The present invention relates to a metering pump for liquid and gaseous media and more especially to a metering pump comprising a deformable tube for the medium to be conveyed, which is disposed in a pump casing and is squeezed against an inside surface in the pump casing by a rotating member.
Elastic tube pumps of this kind are primarily used to convey acid and alkaline media, in particular alkali metal hydroxide solutions and acids which may be in a liquid or gaseous state, abrasive materials and substances of different viscosities, particularly highly viscous substances. These elastic tube pumps act as self-aspirating metering pumps, without valves and seals, and they enable a reversal of the conveying direction by changing the direction of rotation of the driving motor.
In a prior art elastic tube pump, the tube is guided around an angle of 120.degree. inside the pump, between its inlet and outlet side. Within the pump casing, three rollers are provided which force the tube against a plastic-coated inside surface of the casing. The content of the tube is thereby pressed in the conveying direction, according to the rotary motion of the rollers which are disposed at the corners of a rotating triangular-shaped body. Behind each roller a vacuum is produced which exerts a constant suction force upon the material to be conveyed so that the latter is automatically aspirated. For driving the pump, a reduction gear unit can be used, which is actuated by a motor via a V-belt drive. Furthermore, the pump can be coupled directly with a geared motor or it can be driven by a motor via an infinitely variable speed transmission.
In another known elastic tube pump, two rollers are disposed in the pump casing, one close to the inlet and the other close to the outlet of the pump. Both rotate counter-clockwise and are interconnected by struts. The two rollers lie on a common straight line and enclose an angle of 180.degree.. When this arrangement rotates, one of the two rollers is always in contact with the inner wall of the tube, squeezing it together and producing a vacuum behind the roller so that the material to be conveyed is aspirated.
An elastic tube pump which is also known from the prior art is constructed similarly to the above pump. This pump is equipped with two rotating rollers which are arranged opposite to one another and alternatingly compress a tube, squeezing it against a concentric guide channel inside of the pump casing. Similar to the construction in a centrifugal governor, the rollers are attached by brackets to a rotating shaft which is driven from the outside. In this pump, as in the case of the aforementioned pump, the tube is guided around an angle of 180.degree. inside the casing, between the inlet and the outlet side of the pump.
In reproduction technique, diaphragm pumps controlled via pulse generators have, up to now, been used as the primary means to convey a developing medium, for example, an ammonia solution. These pumps have the disadvantage that a surplus of ammonia developing solution is conveyed and introduced into the developing chamber, which surplus serves to bridge an interval between two successive pulses, for example 5 seconds. As a result, the material to be developed may, occasionally, be developed with too much moisture and may become patchy. It is further a disadvantage that the ammonia consumption is generally too high, i.e., it exceeds the quantity needed by about 30 to 40%. Of course, this surplus must be removed again from the developing chamber, and this is not only a time-consuming procedure, but requires also suitable measures to achieve absorption, in order to reduce the proportion of ammonia in the exhaust air. In addition, the surplus quantity collects in the pump body during the down-time of the pump, and this may lead to a failure of the pump.
The aforementioned known metering pumps which are based on elastic tube pumps have the disadvantage that the pump casing contains at least two rotating rollers or other rotating members which compress the tube and axially flex or stretch it at the same time, so that the service life of the tube is reduced and the tube must frequently be exchanged. Since the tube is compressed by at least two rotating members and the circumferential angle of the tube is maximally only 180.degree., a continuous conveyance is not ensured, because when changing from one rotating member to the next, a pulse of pressure is brought to bear upon the tube and, as a result, a slight unsteadiness in the uniform conveyance will always occur.